Bovine heat shock promoter and uses thereof

ABSTRACT

A novel expression system using the heat-inducible bovine hsp70A promoter and associated cis-acting elements is disclosed. The system provides for the continuous production of a highly pure, authentic protein, substantially free of infectious viral and cellular protein contaminants.

This application is a continuation of U.S. patent application Ser. No.08/599,825 filed on Feb. 12, 1996, U.S. Pat. No. 5,733,745, which is acontinuation of application Ser. No. 07/975,719 filed on Nov. 10, 1992,now U.S. Pat. No. 5,521,084.

TECHNICAL FIELD

The present invention relates generally to recombinant gene expressionsystems. More particularly, the invention relates to novel methods forexpressing and secreting gene products using the inducible bovine heatshock promoter. The invention is particularly useful for the productionof pharmaceutically important polypeptides.

BACKGROUND OF THE INVENTION

Proteins are conveniently produced in a variety of procaryotic andeucaryotic recombinant expression systems. These systems, however, oftenfail to mimic natural production such that the resulting protein lacksthe authentic tertiary conformation and post-translational modificationsnormally present. Furthermore, expression levels are frequentlyinadequate, particularly in virally-vectored mammalian systems. Forexample, in lytic systems, expression can be severely limited by lyticfunctions of the virus. When high expression levels are achieved,problems with cell growth and expansion can be encountered due to thecytotoxicity of the expressed proteins. Non-lytic systems often sufferfrom low yields, clone instability and cytotoxicity of the finalproduct.

Inducible expression systems have been employed in an effort to overcomesome of these problems. However, most of the inducible promoterscurrently used in such systems are either restricted to a relativelynarrow range of host cells, are only partially inducible or are derivedfrom organisms, such as tumor viruses, which are inherently dangerous.Accordingly, an inducible expression system which provides for the largescale synthesis of proteins, without the above-described concomitantproblems, would be highly desirable.

One such candidate is a system using a promoter derived from a group ofproteins known as the heat shock proteins (hsps). These proteins areubiquitous, being found in all eucaryotic organisms studied to date, andare inducible by heat stress, as well as a variety of other externalagents. Thus, cells respond to these inducers, such as elevated growthtemperatures, by synthesizing high levels of hsps and coordinatelyreducing the rate of synthesis of other cellular proteins.

Hsps are divided into several groups on the basis of size of interest isthe hsp70 family, so named because these proteins are approximately 70kDa in mass. The level of synthesis of hsp70 in cells during heat shockappears to be linearly related to their thermotolerance. Li, G. C.(1985) Int. J. Radiat. Oncol. Biol. Phys. 11:165-177. Two human hsp70proteins have been described--hsp70A (Wu, B., et al. (1985) Mol. Cell.Biol. 5:330-341; Hunt, C., and Morimoto, R. I. (1985) Proc. Natl. Acad.Sci. USA 82:6455-6459) and hsp70 B (Schiller, P., et al. (1988) J. Mol.Biol. 203:97-105). For a review of hsps, see, e.g., Morimoto et. al.,eds., Stress Proteins in Biology and Medicine (1990) Cold Spring HarborPress; Hightower, L. E. (1991) Cell 66:191-197.; Craig, E. A., andGross, C. A. (1991) Trends Bioch. Sci. 16:135.

The hsp70 promoter, as well as sequences in the 5'- and 3'-untranslatedregions of hsp70 gene transcripts, are responsible for regulating thelevel of protein and mRNA synthesis in the cell in both the induced anduninduced states (Simcox, A. A., et al. (1985) Mol. Cell. Biol.5:3397-3402; Theodorakis, N. G., and Morimoto, R. I. (1987) Mol. Cell.Biol. 7:4357-4368; Yost, H. J., et al. (1990) in Stress Proteins inBiology and Medicine, Morimoto et. al., eds., Stress Proteins in Biologyand Medicine (1990) Cold Spring Harbor Press, at 379-409). A regionknown as the heat shock element (HSE), is found within the first 100 bp5' of the RNA start site of eucaryotic heat shock genes. Sorger, P. K.(1991) Cell 65:363. This region includes the sequence nGAAn, repeated atleast two times in head-to-head or tail-to-tail orientation (nGAAnnTTCnor nTTCnnGAAn). Hsp70 genes from different species differ in the numberand orientation of HSEs and in the types of other factor-binding sitesfound upstream. The HSE functions in stress induced promoter activationby binding a positive transactivating factor, the heat shock factor(HSF). The binding constant of this factor to the heat shock element isabout a hundred fold higher than that of any other known mammaliantranscription factor to its respective binding site, rendering thispromoter one of the strongest.

Hsp promoters have been used to express a variety of genes. For example,Dreano, M., et al. (1986) Gene 49:1-8, describe the use of the humanhsp70B promoter, as well as a Drosophila hsp70 promoter, to direct theheat regulated synthesis of human growth hormone, chicken lysozyme and ahuman influenza haemagglutinin.

EPA Publication No. 336,523 (Dreano et al., published Oct. 11, 1989)describes the in vivo expression of human growth hormone using a humanhsp70 promoter.

PCT Publication No. WO 87/00861 (Bromley et al., published Feb. 12,1987) describes the use of human and Drosophila hsp promoters having5'-untranslated region variants.

EPA Publication No. 118,393 (Bromley et al., published Sep. 12, 1984)and PCT Publication No. WO 87/05935 (Bromley et al., published Oct. 8,1987) describe the expression of E. coli β-galactosidase and humaninfluenza haemagglutinin, using a Drosophila hsp70 promoter.

However, none of the above-described references pertains to bovine hsppromoters or to the use of these promoters to drive the expression ofheterologous proteins in thermotolerant cells. Nor do any of thesereferences describe the use of an hsp70A promoter for recombinantexpression.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention provides a highly efficient inducibleexpression system for the production of recombinant proteins. The systemallows prolonged, reversible production of proteins which mimicauthentic molecules, free of potentially pathogenic agents, in large,economically useful quantities.

In one embodiment, the invention is directed to an isolated bovine hsp70promoter capable of directing the transcription of a heterologous codingsequence positioned downstream therefrom.

In another embodiment, the subject invention is directed to an isolatedbovine hsp70 5'-untranslated region.

In still another embodiment, the invention is directed to a recombinantexpression construct effective in directing the transcription of aselected coding sequence. The expression construct comprises:

(a) bovine hsp70 control sequences; and

(b) a coding sequence operably linked to the control sequences, wherebythe coding sequence can be transcribed and translated in a host cell,and at least one of the control sequences is heterologous to the codingsequence.

In particularly preferred embodiments, the bovine hsp70 controlsequences in the expression construct comprise a nucleotide sequencesubstantially homologous and functionally equivalent to the sequencedepicted at nucleotide positions 1 to 666, inclusive, of the upperstrand of FIG. 2 (SEQ ID NOS:1-2).

Still further embodiments of the subject invention include host cellstransformed with these constructs and methods of producing recombinantpolypeptides using the host cells.

These and other embodiments of the present invention will readily occurto those of ordinary skill in the art in view of the followingdisclosure, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a map of a bovine genomic hsp70 gene λ clone and derivedplasmid. FIG. 1a shows the restriction map of the genomic insert in theλ EMBL3A clone. FIG. 1b shows the BglII-XhoI fragment subcloned inpBLUESCRIPT (pBS). The region indicated by the open bar is the regionsequenced and shown in FIG. 2. The position of the ATG initiation codonof the hsp70 gene is indicated.

FIG. 2 (SEQ ID NOS:1-2) shows a comparison of the sequence of the5'-upstream region and a part of the coding region of the bovine hsp70Agene (numbered on the right) with the human homolog (numbered on theleft). The bovine sequence corresponds to that marked by the open bar inFIG. 1b. The human hsp70A sequence corresponds to bases 40-573 of thesequence published by Hunt, C., and Morimoto, R. I. (1985) Proc. Natl.Acad. Sci. USA 82:6455-6459. The first 40 bp of the human sequence donot significantly match any of the bovine sequence yet determined.Transcription factor binding sites, TATAA box, and translation startcodon are indicated.

FIG. 3 demonstrates the heat regulated expression and secretion ofbovine herpesvirus type 1 ("BHV-¹ ") glycoproteins intransiently-transfected Madin-Darby bovine kidney ("MDBK") cellsanalyzed by Western blotting. FIG. 3a shows an experiment with plasmidp3KHSPG3HU, expressing truncated BHV-1 glycoprotein III ("gIII"). FIG.3b shows an experiment with plasmid p3KHSPG4HU, expressing truncatedBHV-1 glycoprotein IV ("gIV").

FIG. 4 shows the secretion of BHV-1 gIV by a heat inducible clone(MG4-57) of stably-transformed MDBK cells over a protracted time period.FIG. 4a is a depiction of a Coomassie blue-stained gel of culturemedium. FIG. 4b shows a quantitative ELISA determination of gIV proteinin media from successive daily collections plotted cumulatively.

DETAILED DESCRIPTION OF THE INVENTION

The practice of the present invention will employ, unless otherwiseindicated, conventional techniques of molecular biology, microbiology,virology, recombinant DNA technology, and immunology, which are withinthe skill of the art. Such techniques are explained fully in theliterature. See, e.g., Sambrook, Fritsch & Maniatis, Molecular Cloning:A Laboratory Manual, Second Edition (1989); DNA Cloning, Vols. I and II(D. N. Glover ed. 1985); Oligonucleotide Synthesis (M. J. Gait ed.1984); Nucleic Acid Hybridization (B. D. Hames & S. J. Higgins eds.1984); Animal Cell Culture (R. K. Freshney ed. 1986); Immobilized Cellsand Enzymes (IRL press, 1986); Perbal, B., A Practical Guide toMolecular Cloning (1984); the series, Methods In Enzymology (S. Colowickand N. Kaplan eds., Academic Press, Inc.); and Handbook of ExperimentalImmunology, Vols. I-IV (D. M. Weir and C. C. Blackwell eds., 1986,Blackwell Scientific Publications).

All patents, patent applications and publications cited herein, whethersupra or infra, are hereby incorporated by reference in their entirety.

As used in this specification and the appended claims, the singularforms "a," "tan" and "the" include plural references unless the contentclearly dictates otherwise.

A. Definitions

In describing the present invention, the following terms will beemployed, and are intended to be defined as indicated below.

A "thermotolerant cell or cell line" is a cell or cell line obtainedfrom an organism with a normal body temperature above 37° C. It has beenshown that thermotolerance of cultured cells is related to the normalbody temperature of the species from which they are derived. Raaphorst,G. P., et al. (1979) Cancer Res. 39:396. Generally such cells cansurvive and divide at temperatures above 37° C., for a number of hoursand still maintain growth rates substantially the same as rates seenwhen the same cell is grown at 37° C.

By "bovine hsp70 promoter" is meant a DNA regulatory region derived froma bovine hsp70 gene which is capable of binding RNA polymerase andinitiating transcription of a downstream (3'-direction) coding sequence.A "bovine hsp70 promoter" encompasses both promoters with identity to anhsp70 promoter isolated from a bovine species, as well as one which issubstantially homologous and functionally equivalent thereto (as definedbelow). The human and Drosophila hsp70 promoters are specificallyexcluded from this definition. For purposes of defining the presentinvention, the promoter sequence is bound at the 3'-terminus by thetranscriptional start site (but does not necessarily include the sitewhich can be provided by the 5'-UTR, described further below). Thetranscriptional start site is approximately 30 bps downstream(3'-direction) from the TATA box. The promoter extends upstream(5'-direction) to include the minimum number of bases or elementsnecessary to initiate transcription at levels detectable abovebackground. Within the promoter sequence will be found protein bindingdomains (consensus sequences), responsible for binding varioustranscription factors and the TATA box for binding RNA polymerase. Thebovine hsp70 promoter will also include one or more heat shock elementsfor binding heat shock factor during heat stress. A bovine hsp70Apromoter sequence, isolated and cloned as described in the examples, isshown in FIG. 2 (SEQ ID NOS:1-2) and appears to include at leastnucleotides 1 to 441 of the figure.

A "bovine hsp70 5'-UTR" refers to an untranslated region of nucleotidesfrom the bovine hsp70 gene, bound at its 3'-end by the ATG codon andextending upstream (in the 5' direction) to the hsp70 transcriptionstart site. As explained above, this site is located approximately 30nucleotides downstream from the TATA box.

Two DNA or polypeptide sequences are "substantially homologous" when atleast about 80% (preferably at least about 90%, and most preferably atleast about 95%) of the nucleotides or amino acids match over a definedlength of the molecule. As used herein, substantially homologous alsorefers to sequences showing identity to the specified DNA or polypeptidesequence. It is to be understood that a sequence of nucleotides or aminoacids "substantially homologous" to a sequence of nucleotides or aminoacids of bovine hsp70 DNA does not encompass the corresponding human orDrosophila hsp70 nucleotide or amino acid sequences. DNA sequences thatare substantially homologous can be identified in a Southernhybridization, experiment under, for example, stringent conditions, asdefined for that particular system. Defining appropriate hybridizationconditions is within the skill of the art. See, e.g., Sambrook et al.,supra; DNA Cloning, vols I & II, supra; Nucleic Acid Hybridization,supra.

A sequence "functionally equivalent" to a bovine hsp70 sequence is onewhich functions in the same manner as the corresponding hsp70 sequence.Thus, a promoter sequence "functionally equivalent" to the bovine hsp70promoter described herein is one which is capable of directingtranscription of a downstream coding sequence above background levels.

A DNA "coding sequence" or a "nucleotide sequence encoding" a particularprotein, is a DNA sequence which is transcribed and translated into apolypeptide in vivo or in vitro when placed under the control ofappropriate regulatory sequences. The boundaries of the coding sequenceare determined by a start codon at the 5'-(amino) terminus and atranslation stop codon at the 3'-(carboxy) terminus. A coding sequencecan include, but is not limited to, procaryotic sequences, cDNA fromeucaryotic mRNA, genomic DNA sequences from eucaryotic (e.g., mammalian)sources, viral RNA or DNA, and even synthetic nucleotide sequences. Atranscription termination sequence will usually be located 3' to thecoding sequence.

DNA "control sequences" refers collectively to promoter sequences,polyadenylation signals, transcription termination sequences, upstreamregulatory domains, enhancers, and the like, untranslated regions,including 5'-UTRs and 3'-UTRs, which collectively provide for thetranscription and translation of a coding sequence in a host cell.

"Operably linked" refers to an arrangement of elements wherein thecomponents so described are configured so as to perform their usualfunction. Thus, control sequences operably linked to a coding sequenceare capable of effecting the expression of the coding sequence. Thecontrol sequences need not be contiguous with the coding sequence, solong as they function to direct the expression thereof. Thus, forexample, intervening untranslated yet transcribed sequences can bepresent between a promoter sequence and the coding sequence and thepromoter sequence can still be considered "operably linked" to thecoding sequence.

A control sequence "directs the transcription" of a coding sequence in acell when RNA polymerase will bind the promoter sequence and transcribethe coding sequence into mRNA, which is then translated into thepolypeptide encoded by the coding sequence.

A "host cell" is a cell which has been transformed, or is capable oftransformation, by an exogenous DNA sequence.

A cell has been "transformed" by exogenous DNA when such exogenous DNAhas been introduced inside the cell membrane. Exogenous DNA may or maynot be integrated (covalently linked) into chromosomal DNA making up thegenome of the cell. In procaryotes and yeasts, for example, theexogenous DNA may be maintained on an episomal element, such as aplasmid. In eucaryotic cells, a stably transformed cell is generally onein which the exogenous DNA has become integrated into the chromosome sothat it is inherited by daughter cells through chromosome replication,or one which includes stably maintained extrachromosomal plasmids. Thisstability is demonstrated by the ability of the eucaryotic cell toestablish cell lines or clones comprised of a population of daughtercells containing the exogenous DNA.

A "heterologous" region of a DNA construct is an identifiable segment ofDNA within or attached to another DNA molecule that is not found inassociation with the other molecule in nature. For example, a sequenceencoding a bovine protein other than an hsp is considered a heterologoussequence when linked to an hsp bovine promoter. Similarly, a sequenceencoding an hsp will be considered heterologous when linked to an hsppromoter with which it is not normally associated. Another example of aheterologous coding sequence is a construct where the coding sequenceitself is not found in nature (e.g., synthetic sequences having codonsdifferent from the native gene). Likewise, a chimeric sequence,comprising a heterologous structural gene and a gene encoding an hsp ora portion of an hsp, linked to an hsp promoter, whether derived from thesame or a different hsp gene, will be considered heterologous since suchchimeric constructs are not normally found in nature. Allelic variationor naturally occurring mutational events do not give rise to aheterologous region of DNA, as used herein.

The term "immunogenic polypeptide" refers to a polypeptide which elicitsantibodies that neutralize viral or bacterial infectivity (depending onthe antigen in question), and/or mediate antibody-complement or antibodydependent cell cytotoxicity to provide protection of an immunized host.An "immunogenic polypeptide" as used herein, includes the full length(or near full length) sequence of the antigen in question, or animmunogenic fragment thereof. By "immunogenic fragment" is meant afragment which includes one or more epitopes and thus elicits antibodiesthat neutralize viral or bacterial infectivity, and/or mediateantibody-complement or antibody dependent cell cytotoxicity to provideprotection of an immunized host. Such fragments will usually be at leastabout 5 amino acids in length, and preferably at least about 10 to 15amino acids in length. There is no critical upper limit to the length ofthe fragment, which could comprise nearly the full length of the proteinsequence, or even a fusion protein comprising fragments of two or moreepitopes. For example, the BHV-1 gIII and gIV immunogenic polypeptidesexemplified herein are fragments lacking the transmembrane bindingdomains of the proteins, thereby facilitating secretion of the expressedproduct.

B. General Methods

The present invention is based on the isolation and characterization ofa bovine hsp70 promoter and the use of this promoter in an expressionsystem for the production of heterologous proteins. The promoter isinducible. Thus, large quantities of desired proteins can berecombinantly produced by subjecting transformed cells to elevatedtemperatures, as well as to other known inducers of the promoter. Thepromoter can be used to direct the transcription of a desired protein ina wide variety of cell types. If desired, a thermotolerant cell line canbe used, thereby increasing production efficiency, as well as thelongevity of the host cell during recombinant production. Cis-actingcontrol elements can be conveniently associated with the bovine hsp70promoter in order to optimize expression of the structural geneassociated therewith. These regulatory elements direct the efficientexpression of the structural gene during heat shock. If proteinsproduced in the system are either naturally secreted or engineered tobe, the transformed cells can survive and produce the protein productfor protracted time periods, further increasing yields. The systemallows for the production of a desired protein in an authenticconfiguration, with authentic post-translation modifications, in arelatively pure form and in economically useful amounts.

The hsp70 promoter of the present invention can be isolated from abovine genomic library using an appropriate probe and cloned for futureuse. Similarly, the sequence can be produced synthetically, based on thesequence depicted in FIG. 2 (SEQ ID NOS:1-2), using known methods ofpolynucleotide synthesis. See, e.g. Edge, M.D., Nature (1981) 292:756;Nambair, et al. Science (1984) 223:1299; Jay, Ernest, J. Biol. Chem.(1984) 259:6311.

For purposes of the present invention, the bovine hsp70A promoter wasisolated by screening a bovine genomic library with a human hsp70Aprobe, as described further below. The promoter appears to include atleast the nucleotides depicted at positions 1 to 441 of FIG. 2 (SEQ IDNOS:1-2). A TATAAA box (presumed to bind transcription factor IID) islocated at positions 436-441 of FIG. 2 (SEQ ID NOS:1-2). Two CCPAT boxes(the binding sites for the CCAAT box-binding transcription factor, CTF)are located at positions 314 and 397, respectively, of the figure. Apurine rich element and GC element (for binding Sp1 factor) is found atposition 408. Three regions including heat shock elements appear to bepresent at positions 3-24, 265-287 and 350-372.

The bovine hsp70 promoter, or a functional portion thereof, can be usedto direct the transcription of a heterologous coding sequence whenoperably linked thereto. The entire promoter sequence need not bepresent so long as at least one heat shock element, as well as thetranscription initiation site and the RNA polymerase binding site, arepresent. Accordingly, a promoter can be engineered to include only thesenecessary sequences. Generally, for use in the present expressionsystem, a sequence of nucleotides substantially homologous andfunctionally equivalent to nucleotides found at about positions 350 to441, encompassing one heat shock element, more preferably about 265 to441, encompassing two heat shock elements, and even nucleotides 1 to 441and regions extending upstream from position 1 and downstream fromposition 441, will be used to direct the transcription of the desiredheterologous coding sequence.

In order to achieve efficient expression using the bovine hsp70promoter, it is desirable to include an hsp70 5'-UTR region in thepresent system. This region is bound at its 3'-end by the ATG codon andextends upstream (in the 5' direction) to the hsp70 transcription startsite. As explained above, the transcription start site is locatedapproximately 30 nucleotides downstream from the TATA box. Thus, thebovine hsp70A 5'-UTR appears to encompass approximately 190 to 200nucleotides upstream of the ATG codon depicted in FIG. 2 (SEQ IDNOS:1-2). This region shows approximately 65% sequence homology to thecorresponding human hsp70A 5'-UTR.

The hsp70 5'-UTR region used need not be derived from a bovine host, butcan be derived from another corresponding eucaryotic gene, such as froma human hsp70 gene, an insect hsp70 gene, such as from Drosophila, orany other eucaryotic hsp70 gene. If the bovine hsp70A promoter is used,it is preferable to use a corresponding hsp70A 5'-UTR (again, notnecessarily from a bovine source). However, 5'-UTRs derived from hsp70 Bgenes will also find use in systems utilizing the hsp70A promoter.

If an homologous 5'-UTR is utilized, it is generally provided as part ofthe isolated bovine hsp70 promoter and associated sequences and nofurther manipulation is necessary. The 5'-UTR can also be syntheticallyproduced, based on known 5'-UTR sequences, and ligated to the hsp70promoter sequence. Similarly, the 5'-UTR can be isolated from, or thepromoter construct added to, a plasmid bearing the 5'-UTR sequence,using restriction enzymes and procedures. Site specific DNA cleavage isperformed by treatment with a suitable restriction enzyme (or enzymes),under conditions which are generally understood in the art, and theparticulars of which are specified by the manufacturer of thesecommercially available enzymes. See, e.g., New England Biolabs, ProductCatalog. If desired, size separation of the cleaved fragments may beperformed by polyacrylamide gel or agarose gel electrophoresis, usingstandard techniques. A general description of size separations is foundin Methods in Enzymology (1950) 65:499-560. The 5'-UTR and promotersequence can then be ligated to each other using known techniques.

Sequences derived from the 3'-UTR, an untranslated region flanking the3'-end of the hsp70 structural gene, can also be used in conjunctionwith the present system and can be placed downstream from the codingregion to increase expression efficiency thereof. The 3'-UTR appears tostabilize mRNA. As with the 5'-UTR, the 3'-UTR need not necessarily bederived from a bovine hsp gene. Rather, the 3'-UTR can come from anycorresponding hsp70 gene or even the gene to be expressed, provided thatthe gene includes a 3'-UTR. The examples herein describe the use of ahuman hsp70A 3'-UTR and a Drosophila 3'-UTR, respectively, incombination with a bovine hsp70A promoter and 5'-UTR, to direct theexpression of a heterologous coding sequence. The 3'-UTR is ligated 3'to the desired structural gene using techniques known in the art.

Markers and amplifiers can also be employed in the subject expressionsystems. A variety of markers are known which are useful in selectingfor transformed cell lines and generally comprise a gene whoseexpression confers a selectable phenotype on transformed cells when thecells are grown in an appropriate selective medium. Such markers formammalian cell lines include, for example, the bacterialxanthine-guanine phosporibosyl transferase gene, which can be selectedfor in medium containing mycophenolic acid and xanthine (Mulligan et al.(1981) Proc. Natl. Acad. Sci. USA 78:2072-2076), and the aminoglycosidephosphotransferase gene (specifying a protein that inactivates theantibacterial action of neomycin/kanamycin derivatives), which can beselected for using medium containing neomycin derivatives such as G418which are normally toxic to mammalian cells (Colbere-Garapin et al.(1981) J. Mol. Biol. 150:1-14). Useful markers for other eucaryoticexpression systems, are well known to those of skill in the art.

Expression can also be amplified by placing an amplifiable gene, such asthe mouse dihydrofolate reductase (dhfr) gene adjacent to the codingsequence. Cells can then be selected for methotrexate resistance indhfr-deficient cells. See, e.g. Urlaub et al. (1980) Proc. Natl. Acad.Sci. USA 77:4216-4220; Rungold et al. (1981) J. Mol. and Appl. Genet.1:165-175.

The above-described system can be used to direct the expression of awide variety of procaryotic, eucaryotic and viral proteins, including,for example, viral glycoproteins suitable for use as vaccine antigens,immunomodulators for regulation of the immune response, hormones,cytokines and growth factors, as well as proteins useful in theproduction of other biopharmaceuticals.

The present system is particularly useful for the production of bovineviral antigens, such as, but not limited to, antigens derived frombovine herpesvirus (BHV-1), bovine viral diarrhea virus (BVDV), bovinerespiratory syncytial virus, bovine rotavirus, bovine coronavirus andbovine parainfluenza virus. A number of protective antigens from theseviruses are known. For example, in the case of BHV-1, the viral envelopeglycoproteins gI, gIII and gIV have been isolated as well asrecombinantly produced and have been shown to be effective protectiveantigens. (See, e.g. Babiuk, L. A., et al. (1987) Virology 159:57-66 andU.S. Pat. No. 5,151,267, for a description of the isolation and cloningof these antigens, respectively, hereby incorporated by reference intheir entirety). Similarly, monoclonal antibody analysis of gp53 fromBVDV indicates that antibodies thereto possess virus neutralizingactivity. Deregt, D., et al. (1990) Can. J. Vet. Res. 54:343-348 and thenucleotide sequence for gp53 is known (Collett, M. et al. (1988)Virology 165:191-199). Accordingly, the present invention provides amethod for efficiently producing these important antigens.

The gene sequences encoding the desired protein can be isolated orobtained recombinantly, using known techniques. Alternatively, DNAsequences encoding the proteins of interest can be preparedsynthetically rather than cloned. The DNA sequence can be designed withthe appropriate codons for the particular amino acid sequence. Ingeneral, one will select preferred codons for efficient expression inthe intended host. The complete sequence is assembled from overlappingoligonucleotides prepared by standard methods and assembled into acomplete coding sequence. See, e.g., Edge (1981) Nature 292:756; Nambairet al. (1984) Science 223:1299; Jay et al. (1984) J. Biol. Chem.259:6311.

It may also be desirable to produce mutants or analogs of the proteinsof interest. Mutants or analogs may be prepared by the deletion of aportion of the sequence encoding the protein, by insertion of asequence, and/or by substitution of one or more nucleotides within thesequence. Techniques for modifying nucleotide sequences, such assite-directed mutagenesis, are well known to those skilled in the art.See, e.g., Sambrook et al., supra; DNA Cloning, Vols. I and II, supra;Nucleic Acid Hybridization, supra.

For purposes of the present invention, it is particularly desirable tofurther engineer the coding sequence to effect secretion of thepolypeptide from the host organism. This enhances clone stability andprevents the toxic build up of proteins in the host cell so thatexpression can proceed more efficiently. Homologous signal sequences canbe used for this purpose with proteins normally found in associationwith a signal sequence. Additionally, heterologous leader sequenceswhich provide for secretion of the protein can be added to theconstructs. Preferably, processing sites will be included such that theleader fragment can be cleaved from the protein expressed therewith.(See, e.g., U.S. Pat. No. 4,336,246 for a discussion of how suchcleavage sites can be introduced). The leader sequence fragmenttypically encodes a signal peptide comprised of hydrophobic amino acids.

The choice of an appropriate leader will depend on the cell type used toexpress the protein. For example, sequences from genes encoding humana-interferon (Maeda et al. Nature (1985) 315:592), humangastrin-releasing peptide (Lebacq-Verheyden et al. (1988) Molec. Cell.Biol. 8:3129), human IL-2 (Smith et al. (1985) Proc. Natl. Acad. Sci.USA 82:8404), mouse IL-3 (Miyajima et al. (1987) Gene 58:273), and humanglucocerebrosidase (Martin et al. (1988) DNA 7:99), will provide forsecretion of a heterologous protein in mammalian cells. These sequencescan also be used to provide for secretion in insect host cells, as canDNA encoding genes for secreted insect or baculovirus proteins, such asthe baculovirus gp67 gene. For expression in bacteria, DNA encodingsuitable signal sequences can be derived from genes for secretedbacterial proteins, such as the E. coli outer membrane protein gene(ompA) (Ghrayeb et al. (1984) EMBO J. 3:2437) and the E. coli alkalinephosphatase signal sequence (phoA) (Oka et al. (1985) Proc. Natl. Acad.Sci. USA 82:7212). (See, also, U.S. Pat. No. 4,336,336). The signalsequence of the alpha-amylase gene from various Bacillus strains can beused to secrete heterologous proteins from B. subtilis (Palva et al.(1982) Proc. Natl. Acad. Sci. USA 79:5582; EPO Publication No. 244,042).Finally, secretion in yeast can be directed by, i.e., the yeastinvertase gene (EPO Publication No. 012,873; and the α-factor gene (U.S.Pat. Nos. 4,546,083, 4,588,684 and 4,870,008).

For some proteins, particularly viral glycoproteins, secretion can beeffected by deleting all or portions of the transmembrane bindingdomains that might be present, thereby eliminating or substantiallydecreasing transmembrane binding and enhancing secretion. Such anapproach is described in, e.g., Motz, M., et al. (1987) Gene58:149-154); and Spaete et al. (1990) J. Virol. 64:2922-2931.

Alternatively, molecules known to aid in the egress of the expressionproduct from the cell can be coexpressed with the desired protein. See,e.g., Hutchinson et al. (1992) J. Virol. 66:2240-2250.

Once the coding sequences for the desired proteins have been prepared orisolated, an expression vector is constructed so that the particularcoding sequence is located in the vector downstream from the bovinehsp70 promoter and the 5'-UTR, if present. Accordingly, the preferredconstructs of the present invention will generally include, in order, abovine hsp70 promoter sequence including a transcription start site, anhsp70 5'-UTR, a consensus sequence coding for translation initiation, agene sequence coding for a desired protein engineered so that theprotein will be secreted, and a 3'-UTR. Additional interveningnucleotide sequences can also be present so long as transcription andtranslation of the desired coding sequence is not disrupted.

The positioning and orientation of the coding sequence with respect tothe control sequences is such that the coding sequence is transcribedunder the direction of the control sequences (i.e., RNA polymerase whichbinds to the DNA molecule at the control sequences transcribes thecoding sequence). Modification of the sequences encoding the particularprotein of interest may be desirable to achieve this end. For example,in some cases it may be necessary to modify the sequence so that it canbe attached to the control sequences with the appropriate orientation;i.e., to maintain the proper reading frame. The control sequences andother regulatory sequences may be ligated to the coding sequence priorto insertion into a vector. Alternatively, the coding sequence can becloned directly into an expression vector which already contains thecontrol sequences and an appropriate restriction site.

The expression vector is then used to transform an appropriate hostcell. A number of mammalian cell lines are known in the art and includeimmortalized cell lines available from the American Type CultureCollection (ATCC), such as, but not limited to, Chinese hamster ovary(CHO) cells, HeLa cells, baby hamster kidney (BHK) cells, monkey kidneycells (COS), human hepatocellular carcinoma cells (e.g., Hep G2),Madin-Darby bovine kidney ("MDBK") cells, as well as others. Similarly,bacterial hosts such as E. coli, Bacillus subtilis, and Streptococcusspp., will find use with the present expression constructs. Yeast hostsuseful in the present invention include inter alia, Saccharomycescerevisiae, Candida albicans, Candida maltosa, Hansenula polymorpha,Kluyveromyces fragilis, Kluyveromyces lactis, Pichia guillerimondii,Pichia pastoris, Schizosaccharomyces pombe and Yarrowia lipolytica.Insect cells for use with baculovirus expression vectors include, interalia, Aedes aegypti, Autographa californica, Bombyx mori, Drosophilamelanogaster, Spodoptera frugiperda, and Trichoplusia ni.

It may be desirable to use a cell line homologous to the species inwhich the protein is to be used, thereby assuring structuralauthenticity and guaranteeing a product free of heterologous interferingcontaminants. Furthermore, it is particularly preferable to use athermotolerant cell line to produce the desired protein when heat isused as the inducing agent. Such a cell can withstand prolonged elevatedtemperatures, allowing induction of the heat shock promoter and theconcomitant production of the desired protein for an extended period oftime without cell death. A number of thermotolerant cell lines are knownin the art and will generally be derived from organisms that have normalbody temperatures above 37° C. Thus, cells derived from bovine species(having a normal body temperature of 39° C.), such as MDBK cells, willfind use in the subject invention, as will cell lines derived fromporcine (having a normal body temperature of 39° C.), muntjac (having abody temperature of 38.5° C.), and other species.

The transformation procedure used depends upon the host to betransformed. Mammalian cells can conveniently be transformed using, forexample, DEAE-dextran based procedures, calcium phosphate precipitation(Graham, F. L. and Van der Eb, A. J. (1973) Virology 52:456-467),protoplast fusion, liposome-mediated transfer, polybrene-mediatedtransfection and direct microinjection of the DNA into nuclei. Bacterialcells will generally be transformed using calcium chloride, either aloneor in combination with other divalent cations and DMSO (Sambrook,Fritsch & Maniatis, Molecular Cloning: A Laboratory Manual, SecondEdition (1989)). DNA can also be introduced into bacterial cells byelectroporation. Methods of introducing exogenous DNA into yeast hoststypically include either the transformation of spheroplasts ortransformation of intact yeast cells treated with alkali cations.

Proteins are then produced by growing the transformed host cells insuitable media and under conditions that will provide for expression ofthe same. Such conditions are known or will readily be apparent to thoseof skill in the art. It has been found that growth in serum-free medium,with changes every 24 hours, provides dramatically increased yields ofproteins. Production of the desired protein is induced by subjecting thetransformed cells to an agent known to induce the hsp promoter. Suchagents include, for example, heat, metal ions, such as Cd, Zn and Cu,azetidine, forskolin, prostaglandin PGA2, adenovirus ElA protein, aminoacid analogs, certain ionophores, ethanol, hydrogen peroxide andinhibitors of mitochondrial function.

A particularly preferred method of induction is the use of heat. Thus,the hsp promoter is induced by increasing the ambient temperature of thecells during growth, generally in the late stationary phase, to atemperature above 37° C. Generally, cells will be maintained at atemperature of 38 to 45° C., more preferably 39 to 44° C. and mostpreferably 40 to 43° C., for a period of 1 to 12 hours, more preferably4 to 6 hours, and most preferably 6 hours. Additionally, temperaturescan be elevated periodically, i.e. for 1 to 10 hours a day, morepreferably 3 to 6 hours a day, for a period of 1 to 21 days or more.Other suitable temperatures and time periods can be readily determinedby one of skill in the art to assure the efficiency of the hsp70promoter, thereby maximizing production levels of the desired product.

The protein is then isolated from the host cells and purified. If theexpression system secretes the protein into growth media, the proteincan be used directly or purified from the media. If the protein is notsecreted, it is isolated from cell lysates. The selection of anappropriate recovery method is within the skill of the art.

The constructs can also be used in gene therapy or nucleic acidimmunization, to direct the production of the desired gene product invivo, by administering the expression constructs directly to a subjectfor the in vivo translation thereof. See, e.g. EPA Publication No.336,523 (Dreano et al., published Oct. 11, 1989). Alternatively, genetransfer can be accomplished by transfecting the subject's cells ortissues with the expression constructs ex vivo and reintroducing thetransformed material into the host. The constructs can be directlyintroduced into the host organism, i.e., by injection (see InternationalPublication No. WO/90/11092; and Wolff et al., (1990) Science247:1465-1468). Liposome-mediated gene transfer can also be accomplishedusing known methods. See, e.g., Hazinski et al., (1991) Am. J. Respir.Cell Mol. Biol. 4:206-209; Brigham et al. (1989) Am. J. Med. Sci.298:278-281; Canonico et al. (1991) Clin. Res. 39:219A; and Nabel et al.(1990) Science 249:1285-1288. Targeting agents, such as antibodiesdirected against surface antigens expressed on specific cell types, canbe covalently conjugated to the liposomal surface so that the nucleicacid can be delivered to specific tissues and cells for localadministration. Following introduction of the expression constructs intothe host organism, the animals can be heat treated to stimulateproduction of the desired protein using, i.e., a ventilated incubator,as described in EPA Publication No. 336,523 (Dreano et al., publishedOct. 11, 1989). Alternatively, animals can be exposed to fever inducingagents or other stressors, in order to induce the hsp promoter.

C. Experimental

Below are examples of specific embodiments for carrying out the presentinvention. The examples are offered for illustrative purposes only, andare not intended to limit the scope of the present invention in any way.

Efforts have been made to ensure accuracy with respect to numbers used(e.g., amounts, temperatures, etc.), but some experimental error anddeviation should, of course, be allowed for.

Materials and Methods

Enzymes were purchased from commercial sources, and used according tothe manufacturers' directions. Radionucleotides and nitrocellulosefilters were also purchased from commercial sources.

In the cloning of DNA fragments, except where noted, all DNAmanipulations were done according to standard procedures. See Sambrooket al., supra. Restriction enzymes, T₄ DNA ligase, E. coli DNApolymerase I, Klenow fragment, and other biological reagents werepurchased from commercial suppliers and used according to themanufacturers' directions. Double stranded DNA fragments were separatedon agarose gels.

Cells and DNA Transfections

Madin-Darby bovine kidney (MDBK) cells (ATCC Accession No. CCL22) werepropagated in minimal essential medium (MEM) supplemented with 10% fetalbovine serum. Transient DEAE-dextran-mediated DNA transfections wereperformed as described by Kriegler (Kriegler, M., (1990) Gene Transferand Expression (Stockton Press)). Stable transfections were performedusing 5 μg of DNA and 40 μg of Lipofectin (Felgner, P. L., et al. (1987)Proc. Natl. Acad. Sci. USA 84:7413-7417) per 4×10⁶ cells.Neomycin-resistant clones (about 20/μg DNA) were selected by growth inthe presence of 666 μg/ml G418 (Fehler, F., et al. (1992) J. Virol.66:831-839, and maintained in 400 μg/ml.

Monoclonal Antibody Analysis of Secreted gIV

Indirect ELISA's were used to determine the yields of gIV as previouslydescribed (van Drunen-Littel-van den Hurk, S., et al., supra). Theantigenic properties of truncated gIV secreted by MDBK cells wereassessed in an indirect ELISA assay using media containing equivalentamounts of gIV from transfected MDBK cells and recombinant vacciniavirus infected BSC-1 cells, serially diluted and adsorbed to plates. Thereactivity of vaccinia-produced and BHV-1-produced full-length gIV hasbeen previously compared and found to be identical (vanDrunen-Littel-van den Hurk, et al. Vaccine (In Press)). Individual ormixed gIV-specific monoclonal antibodies, followed by horseradishperoxidase-conjugated goat anti-mouse IgG were used for detection aspreviously described (van Drunen-Littel-van den Hurk, S., et al. (1984),supra; Hughes, G., et al., supra).

EXAMPLE 1 Cloning and Identification of the Bovine hsp70 Promoter

A total of 3×10⁶ plaques of a bovine genomic library in λ EMBL3A(Frischauf, A. M. et al. (1983) J. Mol. Biol. 170:827) were screened athigh stringency with a probe produced from a human hsp70 cDNA clone,pH2.3 (Wu, B., et al. (1985) Mol. Cell. Biol. 5:330-341). Five positiveclones were selected and amplified. Three of these clones were unstable,since the λ phage was lost upon passage. The two remaining clones wereidentical and therefore likely to be clonally related. One of these wasselected for further analysis. The restriction map of the genomic insertin this clone is shown in FIG. 1a. Southern blot analysis using afragment of the human hsp70 cDNA clone was used to identify a fragmentof the bovine genome clone with homology to the 5' end of the humanhsp70 mRNA. This BglII-XhoI fragment was subcloned into pBS KSII+(Stratagene; Alting-Mees, M. A., and Short, J. M. (1989) Nucleic AcidsRes. 17:9494) for more detailed restriction enzyme analysis andsequencing.

DNA inserts in pBS KSII+ were sequenced as denatured double-strandedtemplates using a T7 sequencing kit (Pharmacia) and ³⁵ S-dATP(Amersham). Reaction products were analyzed by standard procedures. Datawere analyzed and compared to GenBank files with sequence analysissoftware (IBI, Intelligenetics Inc.). All DNA sequences were determinedby reading the template at least once in each direction.

The entire 1315 bp EcoRI-XhoI fragment shown between positions 2000 and3300 in FIG. 1b was sequenced using this procedure. The sequence of thelast 750 bp is shown in FIG. 2 (SEQ ID NOS:1-2) where it is compared tothe corresponding region of the human hsp70A gene. The sequence frompositions 667-750 constitutes a partial open reading frame. Translationof the sequence revealed that 25 of the 28 encoded amino acids areidentical to the first 28 amino acids of human hsp70A protein (Hunt, C.,and Morimoto, R. I. (1985) Proc. Natl. Acad. Sci. USA 82:6455-6459). TheTATAAA box (presumed transcription factor IID binding site) is locatedat position 436. The human and bovine promoter appear to shareapproximately 70% homology over the -79 and +20 positions. The 5'-UTR ofthe bovine mRNA is approximately 200 nucleotides long versus 215 for thehuman hsp70 mRNA and shares 60% homology with the human RNA of thisregion. The general organization of the promoter is similar to the humanhsp70A promoter (Williams, G. T. and Morimoto, R. I. (1990) Mol. Cell.Biol. 10:3125; Abravaya, et al. (1991) Mol. Cell Biol. 11:586-592). Therelative placement of the heat shock elements (heat shock factor bindingsites, HSE) CCAAT box [CCAAT-box binding transcription factor (CTF)binding site], purine-rich element, and GC element (Sp1 factor bindingsite) between positions 315 and the TATAAA element is the same as theplacement of these regions in the human promoter, with 86% sequenceidentity over this region.

The consensus heat shock element is currently defined as three or moreperfect and imperfect repeats of the sequence NGAAN in a head-to-head orhead-to-tail orientation (Lis, J. T., et al., (1990) in Stress Proteinsin Biology and Medicine, Morimoto, R. I., et al., eds. (Cold SpringHarbor Press)). The bovine hsp70 upstream region has three sites thatmeet these criteria. Two occur at position 265-287 and at position350-372 where the bovine and human sequences are almost identical. TheNGAAN repeats in the latter of these two have been shown to be protectedin in vivo footprinting experiments (Abravaya, K., et al., supra). Asecond cluster of NGAAN elements is found in the bovine sequence atposition 3-24 in the region for which there is no corresponding humansequence available. Three of the 5 NGAAN-type repeats in this regionhave the correct spacing with respect to each other for a consensus heatshock element (position 12-24).

These results show that the promoter region of the bovine homolog of thehuman hsp70A gene has been a isolated and sequenced. This gene issimilar to the human hsp70A gene. The first 28 amino acids of the bovinehsp protein show 90% identity with that of the human hsp70 protein. Thepromoter region shows conservation of recognizable transcription factorbinding sites and their spacing relative to one another (FIG. 2 (SEQ IDNOS:1-2)).

EXAMPLE 2 Expression Plasmid Construction For BHV-1 gIII and gIV

The ability of the bovine hsp70A heat shock promoter to direct theexpression of a heterologous protein in a construct containing thebovine hsp70A 5'-UTR and a human hsp70A 3'-UTR, was tested usingsequences encoding secreted forms of two BHV-1 glycoproteins as follows.After locating the translation initiation codon of the hsp70 openreading frame by sequencing, primers were synthesized for a polymerasechain reaction (PCR). The PCR produced a 530 bp promoter fragmentstarting with the SalI site shown in FIG. 1b and terminating with anNcoI site that incorporates the ATG initiation codon of the hsp70A gene.Plasmids with longer upstream regions were made by rejoining thisfragment to upstream hsp70 fragments via the SalI site.

The BHV-1 gIII gene coding for a secreted form of the protein wasobtained from a plasmid where a linker with stop codons in all threereading frames and an SpeI site had been inserted into the SplI siteimmediately upstream of the transmembrane anchor, terminating theprotein at amino acid 465 (Fitzpatrick, D., et al. (1989) Virology173:46-57). A human hsp70A 3'-UTR fragment with SpeI and ClaI terminiwas obtained from plasmid pH2.3 (Wu, B., et al. (1985) Mol. Cell. Biol.5:330-341) by PCR. This was placed behind the truncated gIII codingsequence creating plasmid p3KHSPG3HU. This plasmid contained 3 kb ofbovine hsp70 upstream sequence, including a bovine hsp70 5'-UTR, withthe BHV-1 protein start codons in precisely the same location as that ofthe bovine hsp70 protein.

The DNA fragment coding for a secreted form of BHV-1 gIV was obtained bymodifying the 5'-end to an NcoI site at the start codon and by insertinga 3-frame stop codon linker at the SacII site immediately upstream ofthe transmembrane anchor terminating the protein at amino acid 320(Tikoo, T. K., et al. (1990) J. Virol. 64:5132-5142). This fragment wasused to replace the gIII sequences in p3KHSPG3HU, creating p3KHSPG4HU.

Plasmids for the generation of stably-transfected cell lines wereconstructed by inserting the cassette for the expression ofaminoglycoside phosphotransferase gene from pSV2NEO (Southern, P. J.,and Berg, P., (1982) Molecular and Applied Genetics 1:327-341) into theconstructs described above, immediately behind the human hsp70 3'-UTR,yielding plasmids pG3HUNEO and pG4HUNEO (ATCC Accession Nos. 69075 and69076, respectively), including the coding sequences for truncated gIIIand gIV, respectively.

The DNA backbone for all these constructs was provided by the plasmidpPOL26 (George, H. J., et al. (1987) Biotechnology 5:600-603).

EXAMPLE 3 Expression of BHV-1 gIII and gIV using Plasmids p3KHSPG3HU andp3KHSPG4HU

Transient assays for expression and secretion of the BHV-1 glycoproteinsfrom plasmids p3KHSPG3HU and p3KHSPG4HU, harboring the gIII and gIVgenes, respectively, driven by the bovine hsp70A promoter and 5'-UTR,and a human hsp70A 3'-UTR, were conducted as follows.

MDBK cells were transformed with the above expression constructs usingthe DEAE-dextran method as described in Materials and Methods.Transiently transfected cell cultures were washed twice to remove serumand incubated at either 37° C. or 43° C. in a minimal volume (5 ml/75cm²) of serum-free MEM or OptiMEM I (Gibco). At the end of theincubation period the medium was collected and centrifuged for 5 min at2000×g to remove cells and debris. Medium was dialyzed and lyophilizedto dryness. Samples were denatured, resolved by electrophoresis in 7.5%Miniprotean gels (Bio-Rad), and proteins detected by Western blotting.The primary antibody was a 1:2000 dilution of a pool of monoclonalantibodies against either BHV-1 gIII or gIV proteins (vanDrunen-Littel-van den Hurk, S., et al. (1984) Virology 135:466-479;Hughes, G., et al. (1988) Archives of Virology 103:47-60). The secondaryantibody was horseradish peroxidase-conjugated goat anti-mouse IgG.

The results of transient assays for expression and secretion of BHV-1glycoproteins in MDBK cells are shown in FIG. 3. FIG. 3a showsexpression of truncated gIII from plasmid p3KHSPG3HU. Lanes 1-5represent analysis of cell culture media added to washed cells at thestart of the time interval (described below). Lanes 6-9 representanalysis of 10% of cell extracts. Lanes 1 and 6 show untransfectedcells; lanes 2 and 7: 37° C., 3 h; Lanes 3 and 8: 43° C., 3 h; lane 4:43° C., 3 h, after which cells were washed and subjected to 37° C., 2 h;lanes 5 and 9: 43° C., 6 h. The four prominent marker bands(non-numbered lanes) correspond to 116, 97, 66 and 45 kDa biotinylatedmolecular weight markers detected with avidin-horseradish peroxidase.Basal level synthesis was barely detectable (lane 2) but up-shift to 43°C. produced a significant amount of gIII in the medium (lane 3). Therewas more gIII protein in the medium after 6 h at 43° C. (lane 5) thanafter 3 h, indicating that the process of secretion continued beyond 3 hat 43° C. Secretion appeared to be efficient, since cellular extractsexamined in lanes 6-9 showed no evidence of intracellular accumulationof gIII.

The expression of truncated gIV from p3KHSPG4HU is shown in FIG. 3b.Lane 1 shows purified truncated gIV protein synthesized and secreted ina vaccinia virus expression system. Lanes 2-5 show MDBK cellstransfected with p3KHSPG4HU. Cell culture media was assayed 24 h later.Cells were washed and incubated for 2 h at 37° C. (lanes 2 and 4) or 43°C. (lanes 3 and 5) in medium without (lanes 2 and 3) or with (lanes 4and 5) 2.5 μg/ml Brefeldin A. Cells were preincubated for 1 h in thedrug prior to incubation at 37° C. or 43° C. The rate of incorporationof ³⁵ S-methionine into acid-precipitable material was unaffected byconcentrations of Brefeldin A as high as 8 μg/ml. The four prominentmarker bands (non-numbered lanes) correspond to 116, 97, 66 and 45 kDabiotinylated molecular weight markers detected with avidin-horseradishperoxidase. Again the basal level of synthesis was barely detectable(lane 2). An up-shift to 43° C. for 3 h caused a dramatic increase inthe amount of the protein in the medium (lane 3). Lanes 4 and 5 showthat the presence of gIV in the medium is actually the result ofsecretion and not the detachment and lysis of cells since Brefeldin A, aspecific inhibitor of transport between the endoplasmic reticulum andGolgi apparatus, blocks this process (lanes 4 and 5). The band thatappears immediately above gIV in lanes 2-5 is residual bovine serumalbumin that is not removed by washing the cells.

EXAMPLE 4 Protracted Regulated Expression and Secretion of BHV-1qIII andgIV from MDBX Cells Stably Transfected with Plasmids pG3HUNEO andpG4HUNEO

The expression and secretion of gIII and gIV from plasmids pG3HUNEO andpG4HUNEO (ATCC Accession Nos. 69075 and 69076, respectively), fromExample 2, was tested as follows. These plasmids include the codingsequences for truncated gIII and gIV, respectively, driven by the bovinehsp70A promoter and 5'-UTR, a human hsp70A 3'-UTR and a cassette codingfor aminoglycoside phosphotransferase.

MDBK cells were transfected and treated as described above to generatestable cell lines resistant to G418. Equal numbers of drug-resistantclones were obtained with each construct. After incubation at either 37°C. or 43°, as described, medium from the stably transfected clones wasconcentrated 4-fold for gel analysis (FIG. 4a) or left undiluted (FIG.4b, table 1). A dot-immunoblot assay was conducted on the medium. Of 160neomycin-resistant clones tested, 100 were found to be heat induciblefor glycoprotein synthesis and secretion. The remainder were negative atboth temperatures. Clones varied in the amount of glycoprotein producedas well as in the ratio between basal and induced protein levels. Nodifferences were observed between the distribution of expressionphenotypes for gIII- and gIV-producing clones. The induction propertiesof 7 gIV-producing clones were followed through 80 cell doublings withno noticeable change.

One gIV clone, designated MG4-57, was tested for synthesis and secretionof glycoprotein over a protracted time period. Clone MG4-57 was grown toconfluence in 4% serum containing culture medium in a 150 cm² flask(4×10⁷ cells) and the flask was incubated with 10 ml serum-free mediumat 43° C. for 6 h every day for 8 days denoting 8 cycles. This mediumwas tested for yield of gIV as shown in FIG. 4. The culture produced gIVin a quantity of 75 μg/150 cm² (4×10⁷ cells) for each 6 h period at 43°C. and the ability to synthesize and secrete gIV did not decrease evenafter 8 cycles. Qualitatively similar results were observed in aparallel experiment with a gIII-secreting clone. Subsequent experimentsindicate that lowering the temperature back to 37° C. does not result inan immediate cessation of synthesis of gIV by the MG4-57 cell line.Consequently, best yields are obtained by performing a 6 h, 43° C. heatshock once in every 24 h, keeping the cells in serum-free mediumthroughout, and changing the medium once every 24 h (immediately priorto heat shock). This medium contains gIV at a level of 10-15 μg/ml andthe experiment has been extended as far as 21 days. Total yield is thus21 days×13 mls×10-15 μg/ml=3-4 mg per 150 cm² flask, or 4×10⁷ cells.

These results and those in Example 3 show that the bovine hsp70A gene isfunctional in the heat regulated expression of recombinant BHV-1glycoproteins and thus is not a bovine heat shock cognate or pseudogene.The results also show that the constructs respond, in transienttransfections, to both increases and decreases in temperature.Stably-transfected cell lines made with these constructs show a highproportion of clones with heat regulatable expression.

EXAMPLE 5 Antigenic Authenticity of gIV Produced by Expression PlasmidpG4HUNEO

The truncated gIV secreted by stably-transfected MDBK cells described inExample 4 was reacted with a panel of monoclonal antibodies directedagainst both continuous and discontinuous epitopes of full length gIV(van Drunen-Little-van den Hurk, S., et al. (1984), supra; Hughes, G.,et al. (1988), supra). The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Reactivity of Monoclonal Antibodies with Full-                                 Length and Truncated gIV.                                                          Epitope   Neutralizing                                                                            ELISA Titer.sup.d                                   MAb.sup.a                                                                           Specificity.sup.b                                                                       Activity.sup.c                                                                          VVgIV  VVΔIV                                                                          hspΔIV                          ______________________________________                                        135   Ia        ++        640    1280   1280                                    9D6 Ib + 640 640 1280                                                         3E7 II + 160 160 160                                                          10C2 IIIa ++ 320 320 160                                                      4Cl IIIb + 320 320 160                                                        2C8 IIIc ++ 160 40 80                                                         3Cl IIId ++ 80 80 80                                                          3DpS IV - 1280 640 320                                                      ______________________________________                                         .sup.a Monoclonal antibodies developed by Hughes, G., et al.(1988)            Archives of Virology 103:47-60.                                               .sup.b gIV epitopes assigned by competitive binding assays (Hughes, et        al., supra).                                                                  .sup.c Neutralizing antibody titers determined for ascites fluids.  titer     <100; + titer: >100; ++ titer: >10,000 (Hughes, et al., supra).               .sup.d Antigen titer was expressed as the reciprocal of the highest           dilution of glycoprotein IV giving a reading of at least 0.05 O.D. (492       nm). A 1:10 dilution corresponds to 0.04 μg of glycoprotein IV per         well. VVgIV = affinity purified fulllength gIV produced in vaccinia           virusinfected BSC1 cells and suspended in OptiMEM I medium (GIBCO) (van       Drunen Littelvan den Hurk et al. Vaccine, in press). VVΔIV =            affinitypurified truncated gIV produced in vaccinia  # virusinfected BSC1     cells and suspended in OptiMEM. hspΔIV = truncated gIV, expressed       under Hsp70A promoter and secreted in OptiMEM by transfected MDBK cells. 

The reactivity was not significantly different from that of truncatedand full-length gIV produced in a vaccinia virus expression system. Thelatter two proteins have been compared to authentic full-length BHV-1gIV, field-tested in cattle, and found to be highly protective againstBHV-1 infection (Van Drunen-Littel-van den Hurk, S., et al., Vaccine, InPress).

Several reports suggest that BHV-1 gIV has cytotoxic properties based onobservations that it is very difficult to isolate stably-transfectedcell lines expressing the native form of this protein (Fehler, F., etal. (1992) J. Virol. 66:831-839). Even though the hsp70A promoter showssome basal activity at 37° C. (see Example 3), the cytotoxicity of theproduct was overcome by using a truncated secreted form of the protein.No difference was observed in the number of stable clones and theirinducible expression properties between constructs expressing gIII andgIV. While not wishing to be bound by theory, it is postulated that thecytotoxicity of gIV is either dependent on its location in the membraneor on the maintenance of a configuration that is lost in the truncatedform. However, as demonstrated herein, the reactivity of the proteinwith several monoclonal antibodies is not substantially altered,suggesting that it is an effective vaccine immunogen.

In summary, the expression system has several important practicaladvantages for the production of vaccines. A single 150 cm² flask of4×10⁷ stationary-phase cells can produce approximately 3 to 4 mg ofantigen (about 250 doses) with minimal manipulations involvingtemperature shifts and media collection. Production is still linearafter 8 temperature-shift cycles, so that extension and optimization ofthe protocol will likely improve yields. This result also demonstratesthat the transfected cells can be maintained in stationary phase, forprotracted periods, while fully retaining the capacity to produce therecombinant protein. In comparison, a vaccinia/BSC-1 system based on aconsensus strong late viral promoter, which is one of the mostproductive mammalian-based expression systems available, produced onlytwice this amount of secreted gIV (van Drunen-Little-van den Hurk, inpress, supra).

EXAMPLE 6 Construction of an Alternative BHV-1 Expression PlasmidUtilizing a Drosophila 3'-UTR

An expression plasmid was constructed as above, using the truncatedBHV-1 gIII gene and a Drosophila melanogastor hsp70 3'-UTR, derived frompl73OR (Voellmy, R., et al. (1985) Proc. Natl. Acad. Sci. USA82:4949-4953) in place of the human 3'-UTR. The resulting plasmid,pGIIIADU, comprising the bovine hsp70A promoter and 5'-UTR, thetruncated gIII gene and the D. melanogastor 3'-UTR, was transfected intoMDBK cells as described in Example 3. The cultures were heat shocked at24 hrs post-transfection and media and cell extracts analyzed by Westernblotting, as described in Example 3.

All of the detectable gIII was found in the medium, indicating that theprotein was indeed expressed and secreted. A low basal level ofsynthesis of gIII at 37° C. was also seen. Shifting the cells to 43° C.resulted in a large induction of synthesis and secretion, presumably dueto the efficient induction of transcription and translation driven bythe bovine hsp70A promoter and associated 5'-UTR. Secretion proceededefficiently at 43° C. for at least 3 to 6 hrs. This experiment showsthat the bovine hsp70A promoter and 5'-UTR, along with the D.melanogastor 3'-UTR, effectively direct regulated expression of a BHV-1glycoprotein in transformed cells.

EXAMPLE 7 Heat Regulated Synthesis of E. coli β-galactosidase

The ability of the bovine hsp70A promoter to express a procaryoticprotein was tested using the E. coli Lac Z gene. Plasmids wereconstructed by cloning the SalI-NcoI fragment that incorporates the ATGinitiation codon of the hsp70A gene shown in FIG. 1b, as describedabove, into plasmid pPOL26 (George, H. J., et al., (1987) Biotechnology5:600-603) containing the Lac Z sequence. The coding sequence wasimmediately followed by a translation stop codon and the SV40 small tintron and polyadenylation signal. This construct was introduced intoMDBK cells by DEAE dextran-mediated DNA transfection, as describedabove. β-galactosidase production was monitored histochemically by theaddition of X-gal to fixed cells and by assay of β-galactosidaseactivity in cell extracts. The experiments showed that the bovine hsp70promoter directed the production of amounts of β-galactosidasetwenty-fold higher than those produced by plasmids utilizing the SV40early promoter for the same purpose.

EXAMPLE 8 Expression of BVDV qp53 Using the Hsp70 Promoter

BVDV is a positive stranded RNA virus. Accordingly, the gene was clonedby reverse transcription of RNA prepared from infected cells, followedby a polymerase chain reaction to amplify the DNA. PCR primers weredesigned so as to provide an NcoI site incorporating a start codon infront of amino acid 665 of the NADL strain sequence, and a stop codonand a SalI site after amino acid 1036. This DNA was inserted intopG4HUNEO in place of the BHV-1 gIV coding sequence. The resultingplasmid, designated pGP53HUNEO was transfected into bovine MDBK cells.

Thus, a novel bovine hsp70 promoter and the use of the promoter inefficient expression systems have been described. Although preferredembodiments of the subject invention have been described in some detail,it is to be understood that obvious variations can be made withoutdeparting from the spirit and the scope of the invention as defined bythe appended claims.

Deposits of Strains Useful in Practicing the Invention

A deposit of biologically pure cultures of the following strains wasmade with the international depository authority, American Type CultureCollection, 10801 University Boulevard, Manassas, Va. 20110-2209pursuant to the terms of the Budapest Treaty. The accession numberindicated was assigned after successful viability testing, and therequisite fees were paid. Access to said cultures and informationpertaining thereto will only be given to those authorities, naturalpersons or legal entities entitled therto under Rule 11 of the BudapestTreaty and subject to the conditions as provided in that Rule. The abovecultures were deposited under conditions which assure that access tosaid cultures will be available during pendency of the patentapplication to one determined by the Commissioner to be entitled theretounder 37 CFR 1.14 and 35 USC 122. All restriction on availability ofsaid cultures to the public will be irrevocably removed upon thegranting of a patent based upon the application. The designated depositswill be maintained for a period of at least thirty (30) years from thedate of deposit, or for five (5) years after the last request for thedeposit. Should a culture become nonviable or be inadvertentlydestroyed, or, in the case of plasmid-containing strains, lose itsplasmid, it will be replaced with a viable culture(s) of the sametaxonomic description.

These deposits are provided merely as a convenience to those of skill inthe art, and are not an admission that a deposit is required under 35USC §112. The nucleic acid sequences of these plasmids, as well as theamino sequences of the polypeptides encoded thereby, are incorporatedherein by reference and are controlling in the event of any conflictwith the description herein. A license may be required to make, use, orsell the deposited materials, and no such license is hereby granted.

    ______________________________________                                        Strain           Deposit Date                                                                            ATCC No.                                           ______________________________________                                        pG3HUNEO (in E. coli)                                                                          9/29/92   69075                                                pG4HUNEO (in E. coli) 9/29/92 69076                                         ______________________________________                                    

    __________________________________________________________________________    #             SEQUENCE LISTING                                                   - -  - - (1) GENERAL INFORMATION:                                             - -    (iii) NUMBER OF SEQUENCES: 2                                           - -  - - (2) INFORMATION FOR SEQ ID NO:1:                                     - -      (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 750 base - #pairs                                                 (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: single                                                      (D) TOPOLOGY: linear                                                 - -     (ii) MOLECULE TYPE: DNA (genomic)                                     - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                               - - TCTTCGAGAA ACTCGGGAAC TTTCTGTATT TTGGCTGTCC CGGCAGTCGT GT -            #AGCCCTTA     60                                                                 - - ATTCTACTTT AAACCACCAA ACTAATTTGA GCCCCGAGAT CCTCTCACCG CC -            #CTACAATT    120                                                                 - - AATTACAAGC CCAGGGCTGA TCCTTCCAGT CGACTCGACT CCAAACTACT TG -            #GCTGGCTG    180                                                                 - - GTCGCCAGGA AACCAGAGAC AGAGTGGGTG GACCTTCCCA GCCCCTCTCC CC -            #CTCTCCTT    240                                                                 - - AGGACTCCTG TTTCCTCCAG CGAATCCTAG AAGAGTCTGG AGAGTTCTGG GA -            #GGAGAGGC    300                                                                 - - ATCCAGGGCG CTGATTGGTT CCAGAAAGCC AGGGGGCAGG ACTTGAGGCG AA -            #ACCCCTGG    360                                                                 - - AATATTCCCG ACCTGGCAGC CCCACTGAGC TCGGTCATTG GCTGACGAGG GA -            #AAAGGCGG    420                                                                 - - GGCTTGATGA AGAATTATAA ACACAGAGCC GCCTGAGGAG AAACAGCAGC CT -            #GGAGAGAG    480                                                                 - - CTGATAAAAC TTACGGCTTA GTCCGTGAGA GCAGCTTCCG CAGACCCGCT AT -            #CTCCAAGG    540                                                                 - - ACCGCCCGAG GGGCACCAGA GCGTTCAGTT TTCGGGTTCC GAAAAGCCCG AG -            #CTTCTCGT    600                                                                 - - CGCAGATCCT CTTCACCGAT TTCAGGTTTG AAGCTTATTT CGGAGCCGGA AA -            #AGCAGGGC    660                                                                 - - ACCGGCATGG CGAAAAACAC AGCTATCGGC ATCGACCTGG GCACCACCTA CT -            #CCTGCGTA    720                                                                 - - GGGGTGTTCC AGCACGGCAA GGTGGAGATC         - #                  - #              750                                                                     - -  - - (2) INFORMATION FOR SEQ ID NO:2:                                     - -      (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 533 base - #pairs                                                 (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: single                                                      (D) TOPOLOGY: linear                                                 - -     (ii) MOLECULE TYPE: DNA (genomic)                                     - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                               - - TTCCTCTCAG GGTCCCTGTC CCCTCCAGTG AATCCCAGAA GACTCTGGAG AG -             #TTCTGAGC     60                                                                 - - AGGGGGCGGC ACTCTGGCCT CTGATTGGTC CAAGGAAGGC TGGGGGGCAG GA -            #CGGGAGGC    120                                                                 - - GAAACCCCTG GAATATTCCC GACCTGGCAG CCTCATCGAG CTCGGTGATT GG -            #CTCAGAAG    180                                                                 - - GGAAAAGGCG GGTCTCCGTG ACGACTTATA AAAGCCCAGG GGCAAGCGGT CC -            #GGATAACG    240                                                                 - - GCTAGCCTGA GGAGCTGCTG CGACAGTCCA CTACCTTTTT CGAGAGTGAC TC -            #CCGTTGTC    300                                                                 - - CCAAGGCTTC CCAGAGCGAA CCTGTGCGGC TGCAGGCACC GGCGCGTCGA GT -            #TTCCGGCG    360                                                                 - - TCCGGAAGGA CCGAGCTCTT CTCGCGGATC CAGTGTTCCG TTTCCAGCCC CC -            #AATCTCAG    420                                                                 - - AGCCGAGCCG ACAGAGAGCA GGGAACCGCA TGGCCAAAGC CGCGGCAGTC GG -            #CATCGACC    480                                                                 - - TGGGCACCAC CTACTCCTGC GTGGGGGTGT TCCAACACGG CAAGGTGGAG AT - #C               533                                                                     __________________________________________________________________________

We claim:
 1. An isolated bovine hsp70A promoter capable of directing thetranscription of a heterologous coding sequence positioned downstreamtherefrom, wherein said promoter is selected from the group consistingof (a) a promoter comprising the nucleotide sequence depicted atnucleotide positions 350 to 441, inclusive, of the upper strand of FIG.2 (SEQ ID NO:1); (b) a promoter comprising the nucleotide sequencedepicted at nucleotide positions 265 to 441, inclusive, of the upperstrand of FIG. 2 (SEQ ID NO:1); (c) a promoter comprising the nucleotidesequence depicted at nucleotide positions 1 to 441, inclusive, of theupper strand of FIG. 2 (SEQ ID NO:1); and (d) a promoter comprising anucleotide sequence that hybridizes to the promoter of (a), (b) or (c)in a Southern hybridization reaction performed under stringentconditions.
 2. The promoter of claim 1 wherein said promoter comprisesthe nucleotide sequence depicted at nucleotide positions 350 to 441,inclusive, of the upper strand of FIG. 2 (SEQ ID NO:1).
 3. The promoterof claim 1 wherein said promoter comprises the nucleotide sequencedepicted at nucleotide positions 265 to 441, inclusive, of the upperstrand of FIG. 2 (SEQ ID NO:1).
 4. The promoter of claim 1 wherein saidpromoter comprises the nucleotide sequence depicted at nucleotidepositions 1 to 441, inclusive, of the upper strand of FIG. 2 (SEQ IDNO:1).
 5. An isolated bovine hsp70A 5'-untranslated region.
 6. Arecombinant expression construct effective in directing thetranscription of a selected coding sequence, said expression constructcomprising:(a) bovine hsp70A control sequences comprising a bovinehsp70A promoter according to claim 1; and (b) a coding sequence operablylinked to said control sequences, whereby said coding sequence can betranscribed and translated in a host cell, and at least one of saidcontrol sequences is heterologous to said coding sequence.
 7. Therecombinant expression construct of claim 6 wherein said bovine hsp70control sequences comprise a bovine hsp70A promoter and an hsp705'-untranslated region.
 8. The recombinant expression construct of claim7 wherein said bovine hsp70A promoter comprises the nucleotide sequencedepicted at nucleotide positions 350 to 441, inclusive, of the upperstrand of FIG. 2 (SEQ ID NO:1).
 9. The recombinant expression constructof claim 7 wherein said bovine hsp70A promoter comprises the nucleotidesequence depicted at nucleotide positions 265 to 441, inclusive, of theupper strand of FIG. 2 (SEQ ID NO:1).
 10. The recombinant expressionconstruct of claim 7 wherein said bovine hsp70A promoter comprises thenucleotide sequence depicted at nucleotide positions 1 to 441,inclusive, of the upper strand of FIG. 2 (SEQ ID NO:1).
 11. Therecombinant expression construct of claim 7 wherein said hsp705'-untranslated region comprises a bovine hsp70A 5'-untranslated region.12. A recombinant expression construct comprising:(a) a nucleic acidmolecule that comprises a coding sequence for a selected polypeptide;and (b) hsp70 control sequences operably linked to said nucleic acidmolecule, whereby said coding sequence will be transcribed andtranslated when in a host cell to produce said selected polypeptide, andat least one of said control sequences is heterologous to said codingsequence, wherein said hsp70 control sequences comprise a bovine hsp70Apromoter according to claim 1, a bovine 5'-untranslated regionpositioned downstream of said promoter and a 3'-untranslated regionpositioned downstream of said coding sequence.
 13. The recombinantexpression construct of claim 12 wherein said 3'-untranslated region isan hsp70 3'-untranslated region.
 14. The recombinant expressionconstruct of claim 13 wherein said hsp70 3'-untranslated regioncomprises a human hsp70 3'-untranslated region.
 15. The recombinantexpression construct of claim 13 wherein said hsp70 3'-untranslatedregion comprises a Drosophila hsp70 3'-untranslated region.
 16. Therecombinant expression construct of claim 13 wherein said hsp703'-untranslated region comprises a bovine hsp70 3'-untranslated region.17. The recombinant expression construct of claim 12 wherein said codingsequence encodes an immunogenic bovine herpesvirus type 1 (BHV-1) gIIIpolypeptide.
 18. The recombinant expression construct of claim 12wherein said coding sequence encodes an immunogenic bovine herpesvirustype 1 (BHV-1) gIV polypeptide.
 19. The recombinant expression constructof claim 12 wherein said coding sequence encodes an immunogenic bovineviral diarrhea virus (BVDV) gp53 polypeptide.
 20. A recombinantexpression construct effective in directing the transcription of aselected coding sequence, said expression construct comprising:(a)bovine hsp70A control sequences comprising the nucleotide sequencedepicted at nucleotide positions 1 to 666, inclusive, of the upperstrand of FIG. 2 (SEQ ID NO:1), or a nucleotide sequence that hybridizesthereto in a Southern hybridization reaction performed under stringentconditions; and (b) a coding sequence operably linked to said controlsequences, whereby said coding sequence can be transcribed andtranslated in a host cell, and at least one of said control sequences isheterologous to said coding sequence.
 21. A recombinant expressionconstruct comprising:(a) a nucleic acid molecule that comprises a codingsequence for a selected polypeptide; and (b) hsp70 control sequencesoperably linked to said nucleic acid molecule and a 3'-untranslatedregion positioned downstream of said coding sequence, whereby saidcoding sequence will be transcribed and translated when in a host cellto produce said selected polypeptide, and at least one of said controlsequences is heterologous to said coding sequence, wherein said hsp70control sequences comprise the nucleotide sequence depicted atnucleotide positions 1 to 666, inclusive, of the upper strand of FIG. 2(SEQ ID NO:1).
 22. The recombinant expression construct of claim 21wherein said 3'-untranslated region is an hsp70 3'-untranslated region.23. The recombinant expression construct of claim 22 wherein said hsp703'-untranslated region comprises a human hsp70 3'-untranslated region.24. The recombinant expression construct of claim 22 wherein said hsp703'-untranslated region comprises a Drosophila hsp70 3'-untranslatedregion.
 25. The recombinant expression construct of claim 22 whereinsaid hsp70 3'-untranslated region comprises a bovine hsp703'-untranslated region.
 26. The recombinant expression construct ofclaim 23 wherein said coding sequence encodes an immunogenic bovineherpesvirus type 1 (BHV-1) gIII polypeptide.
 27. The recombinantexpression construct of claim 23 wherein said coding sequence encodes animmunogenic bovine herpesvirus type 1 (BHV-1) gIV polypeptide.
 28. Therecombinant expression construct of claim 23 wherein said codingsequence encodes an immunogenic bovine viral diarrhea virus (BVDV) gp53polypeptide.
 29. A host cell stably transformed with the recombinantexpression construct of claim
 6. 30. A host cell stably transformed withthe recombinant expression construct of claim
 20. 31. A host cell stablytransformed with the recombinant expression construct of claim
 26. 32. Ahost cell stably transformed with the recombinant expression constructof claim
 27. 33. A host cell stably transformed with the recombinantexpression construct of claim
 28. 34. The host cell of claim 29 whereinsaid cell is a thermotolerant cell.
 35. The host cell of claim 30wherein said cell is a thermotolerant cell.
 36. The host cell of claim34 wherein said thermotolerant cell is a Madin-Darby bovine kidney(MDBK) cell.
 37. The host cell of claim 35 wherein said thermotolerantcell is a Madin-Darby bovine kidney (MDBK) cell.
 38. A method ofproducing a recombinant polypeptide comprising:(a) transforming athermotolerant mammalian host cell with a recombinant expressionconstruct according to claim 12, and (b) treating said transformed cellwith heat, under conditions whereby said coding sequence for saidselected polypeptide is expressed, thereby producing said recombinantpolypeptide.
 39. A method of producing a recombinant polypeptidecomprising:(a) transforming a thermotolerant mammalian host cell with arecombinant expression construct according to claim 21; and (b) treatingsaid transformed cell with heat, under conditions whereby said codingsequence for said selected polypeptide is expressed, thereby producingsaid recombinant polypeptide.